Magnetic disks are data memories with a very large capacity. Millions of characters (bytes) can be stored on a magnetic disk about 95 mm in size.
In addition to high storage density, the disks must also have exact mechanical and tribological characteristics. In some applications, the disks move at a speed of approx. 5400 rotations per minute. This means that the outer edge of the disk is subjected to a speed of up to 100 km/h, in which the write/read head is only a few ten thousandths of a millimeter from the surface of the disk.
Only the greatest precision in manufacture and statistical process controls can achieve these quality requirements.
Since hard disk drives with low overhung magnetic heads have been manufactured, these hard disks have been equipped with a lubricant to prevent friction, abrasion and damage (tribological effects). The lubricant, generally a linear or branched perfluorinated polyether, is first applied to the disk, in order to ensure even surface distribution. Such an arrangement is known through DE- AS-28 39 378 and EP-A-0 123 707, for example.
Other systems are based on a selective vapour pressure reservoir, in which an additional vapourisable lubricant is introduced into the drive in order to achieve condensation from the vapour phase onto the critical areas.
If a magnetic head is now applied to a disk lubricated in this way and the drive started, the head is "contaminated" by the lubricants (lube) on the outer layer of the disk through molecular transfer from the disk to the head, which thus receives a layer of lubricant.
Within such an arrangement of magnetic head and disk to the hard disk interface, the air streaming past also comes across two fluorinated surfaces. Due to the autophobic character of the lube layer, the energy transferred in a chance contact between the head and the disk is reduced to a minimum.
The lubrication of the magnetic head too has the additional advantage in that the lubricant works as a chemical surface protector, as the affinity of organic materials, potential deposits and condensed products are greatly reduced.
U.S. Pat. No. 5,409,738 describes a recording medium consisting of a substrate, a thin film for picking up corresponding data, a protective layer on the magnetic layer as well as a layer of lubricant arranged on this protective layer. Through oxidative surface polymerisation in the plasma, this represents a manufactured product in which main molecule chains are chemically linked to the protective layer. In this way, impurities such as ho water or organic bonds getting between the disk and the lubricant can be avoided. This process has the disadvantage that on the surface of the substrate an additional discrete layer is created, which represents an additional interface and which can very easily contaminate the magnetic head. Wetting of the magnetic head with the lubricant does not take place.
In newer hard disk drives, the distance between head and disk is even smaller, in order that a higher storage density can be achieved. Naturally, even greater demands are made of the tribological characteristics of the interface between magnetic disk and head. To improve the surface hardness, a carbon layer which resembles diamond (carbon overcoat) is applied to the substrate surface, to which the lubricant is applied as a final layer. In this way, abrasion can be reduced and a smooth layer is achieved on the magnetic disk.
An additional "desired" lubrication of the magnetic head was not previously necessary, as the lube was applied automatically through the operation of the hard disk drive. It has been found that deliberate pre-lubrication of the magnetic head increases a problem already known to exist, known as stiction.
Whether the critical surface of the magnetic head is wetted first by the lube or by contamination depends, amongst other things, on the adsorption ratios. Generally, in such competitive processes, one is dealing with exclusion processes, i.e. should the pick up of contaminant droplets (e.g. oils) predominate, then the wetting with lube is all the more unlikely, because the surface has then changed its affinity more strongly towards picking up oils. The process tips in one direction or the other. This is also called autocatalytic affinity.
Should oil droplets be taken up, then they will accumulate on the back edge of the magnetic head through the flow of air. When the head has stopped moving on the disk, then this will be drawn by capillary attraction along the underside of the magnetic head in the gap between the head and the disk, and will thus stick the magnetic head onto the disk (glass disk effect). If the head does not move for a sufficiently long length of time and the sticking effect is correspondingly strong, this phenomenon can lead to the magnetic head being torn from its holder and to irreparable damage being caused to the magnetic disk.
Due to the increasing optimization of the interface towards greater storage density and smaller write/read distances, it now appears that a limit has been reached, in that the lube in the course of the initial use of a disk drive is no longer being transferred quickly enough from the disk to the head. Recent analytical observations on the part of the applicants have shown that dependent on selected write/read distances, also under conditions of several weeks of running, no traces of lube and almost no closed layers exist on the heads. On the other hand, in the first hours of running all types of contaminants such as plasticizers, outgassing products of adhesives and elastomers etc. were measured, which could be damaging for the operation of the drive and lead to failures due to stiction.
As no lube layer can be introduced directly onto the read head due to stiction problems described above, and on the other hand as a lack of protective layer leads to pick up of contaminants and thus to renewed stiction, the necessity exists of developing a process which allows a layer of lubrication to be applied to the carbon overcoat of the magnetic head without simultaneously having to accept the disadvantages of stiction effects. In addition, no further layer should have to be brought onto the magnetic head, as this would be soft in comparison to the hard carbon overcoat layer and thus could again occasion further contamination.